1. Field of the Invention
The invention relates to a strip transmission filter with resonators that are provided in a ceramic substrate. In particular, the invention relates to a strip transmission filter having capacitive coupling structures for coupling an HF signal in and out, and in which the ceramic substrate has a metallic coating on all sides with the exception of the face side, which has the coupling structures.
Known strip transmission filters typically have a capacitive coupling of the resonators. On the base surface of a ceramic substrate, one surface is separated from the surrounding mass galvanically, so that this surface forms a capacitance with repsect to a track that is arranged above on the ceramic substrate and is separated by the dielectric.
The capacitance is dependent on the dielectric constant xcex5 of the dielectric, the thickness of the substrate, and the size of the surface.
European Document No. 718 906 A1 teaches a ceramic substrate having two strip transmission line resonators arranged on its top surface. By a galvanic separating of the all over ground metallization, metal surfaces for coupling are created on the bottom side of the substrate. On the top side of the ceramic substrate with the stripline resonators, metallic surfaces are arranged, which are contacted to the metallic surfaces for coupling with the aid of through holes which are metallized on the inside.
The coupling of the strip transmission filter is accomplished by contacting the metallic coupling surfaces, which are galvanically separated from the surrounding ground metallization by the separating surfaces, through to the other side of the ceramic substrate. Thus, the capacitive coupling with the strip transmission resonators on the opposite side is achieved.
The coupling is determined mainly by the spacing of the coupling structures, and not by the thickness of the substrate, which preferably consists of a highly dielectric microwave ceramic. Thus, the coupling structure can be produced mechanically or by etching with significantly higher tolerances. Furthermore, the coupling can also be created by etching, whereby the position of the mask of photosensitive resist is not critical to the coupling capacity.
A disadvantage of the above strip transmission filter is that, it can be produced only at a relatively great expense. Another disadvantage is that the dimensions of the component are also increased due to the measure of through-contacting that is taken in order to connect the metallic surfaces that are provided on both surfaces of the strip transmission filter to each other.
Furthermore, the pass characteristic of microwave ceramic filters should have an optimally high edge steepness in order to suppress LO oscillator frequencies, image frequencies, etc. At the same time, however, the pole numberxe2x80x94that is, the number of inner conductorsxe2x80x94should be optimally small in order to obtain an optimally low insertion loss (which will be detailed later), since this rises with the number of poles.
Microwave ceramic filters were used for this, in which the coupling structures are arranged in the region of the outer edges, and a one-sided steepening of the filter curve edges was achieved by impedance skips of xcex/4 resonators that are to be coupled to one another.
It is the an object of the present invention to produce a strip transmission filter which comprises a better adhesion of the metallic coating on the ceramic substrate. A further object is to be able to adapt the bandwidth of the filter to the respective applications without notable outlay.
In an embodiment, the present invention provides a strip transmission filter having a ceramic substrate having a face side. A metallic coating is on the ceramic substrate, but not on the face side of the substrate. Resonators are provided in the ceramic substrate, the resonators each have an impedance skip. Capacitive couplings are on the face side and provide a HF signal in and out of the strip transmission filter. At least one ground terminal is on the face side. The ceramic structure is stepped in a region of the capacitive couplings and the at least one ground terminal.
In an embodiment, the strip transmission filter includes a step depth of the at least one ground terminal that can be adjusted for the purpose of changing a bandwidth of the filter.
In an embodiment, the strip transmission filter has an edge steepness that provides a filter curve, F. The filter curve, F, is adjustable by changing a spacing, A, between the capacitive couplings and a size of each of the capacitive couplings.
According to a preferred embodiment of the invention, a gradation is realized in the ceramic substrate in which the strip transmission resonators of the strip transmission filter are arranged, said step being realized in the region of the capacitively acting coupling structures and in the region of at least one ground terminal.
Such an inventively provided step in the region of the coupling structures increases the adhesion of the metallic coating, and allows for more effective soldering of microwave lines that are to be connected through this.
According to an advantageous development of the invention, the depth of the stepxe2x80x94that is, the dimension of the step in the direction of the longitudinal extent of the striplinesxe2x80x94of a ground terminal that is provided in the region of the coupling structures can be adjusted. This makes it possible to correct the bandwidth of the pass characteristic of the strip transmission filter in a simple but effective manner.
Furthermore, according to another advantageous development of the invention, the edge steepness of the filter curve can be correspondingly adjusted in view of the respective application by varying the distance between the coupling structures and/or varying the size of the coupling structures.
In practice, this means that when the spacing between the coupling structures is small and the coupling itself is selected to be weak (for instance, by constructing the coupling structures very narrow), the edge steepness of a strip transmission filter of such a construction is significantly increased due to an overcoupling between the coupling structures.
The invention is detailed below with reference to the enclosed drawings.